Re: [boost] Using BOOST_ASSERT to check numeric_limits<FPT>::epsilon

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-02-11 09:34:06

| -----Original Message-----
| From: boost-bounces_at_[hidden] 
| [mailto:boost-bounces_at_[hidden]] On Behalf Of John Maddock
| Sent: 11 February 2006 13:43
| To: boost_at_[hidden]
| Subject: Re: [boost] Using BOOST_ASSERT to check 
| numeric_limits<FPT>::epsilon
| 
| > epsilon should be a constant known at compile time?
| > But is the compile unable to know this?
| 
| Static assertions require an integral-constant expression, 
| and epsilon is 
| basically a runtime constant (at best it's a floating point constant).
| 
| BTW I don't believe you can portably use DBL_EPSILON etc in a static 
| assertion: you certainly can't use the DBL_* macros in preprocessor 
| directives as I've found to my cost already.  A vendor is entitled to 
| implement this as:
| 
| #define DBL_EPSILON __dbl_eps
| extern const double __dbl_eps;
| 
| > Other suggestions on how to achieve a compile-time warning here?
| 
| Not really, other than checking for numeric_limits<>::is_specialized.
| 
| John. 

I feared as much.  (In fact worse than I feared - you might like to add
these to the static assert docs - I would have dropped into this trap :-((
)

Supplementary question:

What impact will it each of the following have on Darwin long double 

(that I believe you have used)

if fp comparisons includes:
BOOST_STATIC_ASSERT(numeric_limits<FPT>::is_specialized);

and

BOOST_STATIC_ASSERT(numeric_limits<FPT>::is_iec559);

and

if (numeric_limits<FPT>::epsilon() <= (FPT)0)
{ // numeric_limits not correctly specilized!
        return false; // Better warning desirable. Throw?
}

We discussed numeric_limits<FPT>::epsilon() and I think we can produce an
intuitively useful (if not exact) value using the normal formula
2^(1-significands)

Paul

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Paul A Bristow
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